where \(U_\mathrm {eq}\)  is the open-circuit potential of the cell, \(\eta _\mathrm {r}\)  and \(\eta _\mathrm {c}\)  are the potential drops due to the reaction and concentration overpotentials, respectively; and \(\Delta \phi _\mathrm {e}\)  and \(\Delta \phi _\mathrm {s}\)  are the Ohmic losses in the electrolyte and the electrodes, respectively. During discharge (\(i_\mathrm {app} > 0\) ), as lithium ions flow from the negative to positive electrode, the quantities \(\eta _\mathrm {r}\) , \(\eta _\mathrm {c}\) , \(\Delta \phi _\mathrm {e}\)  and \(\Delta \phi _\mathrm {s}\)  are all positive, and represent the reduction in the open circuit cell potential resulting from these loss mechanisms. During charge (\(i_\mathrm {app}<0\) ) these quantities are all negative and therefore represent the extra potential, above the open-circuit potential, required to overcome the internal resistances in the cell. This decomposition of the voltage into constituent parts naturally appears when the model is derived (see, for example, [1]}), thus providing further physical intuition into what effects govern the voltage response of the battery.
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